The aim of the research proposed in this application is to gain insight into the molecular mechanism of signal transduction in the visual system. It is expected that these studies will contribute to a better understanding of the relationship between receptor structure, function, and information processing in biological systems. The genes encoding four Drosophila opsins have been isolated and characterized (Rh1-4). Each of these is expressed in distinct classes of photoreceptor cells and displays characteristic patterns of sensitivity to light. The Rh1 opsin is a blue sensitive pigment (lambda max 480 nm) which is expressed in the six outer photoreceptor cells (R1-R6). Rh2 displays light sensitivity with lambda max 420 nm and is expressed in the occelli. Rh3 and Rh4 are expressed in non-overlapping sets of UV sensitive R7 cells, and appear to correspond to the yellow and pale pigments expressed in these cells. In previous studies, a Drosophila mutant lacking the wildtype Rh1 gene product (NinaE) was transformed with a chimeric gene composed of the Rh1 promoter and the Rh2 structural gene. In these flies, the Rh2 opsin was misexpressed in R1-R6 cells and conferred new spectral and physiological characteristics to this class of photoreceptor cells. These studies demonstrate the feasibility of targeting gene constructs to specific photoreceptor cells for expression and analysis, and provides an ideal system for studying spectral tuning of Drosophila opsins in vivo. In an attempt to determine the molecular basis of spectral specificity and sensitivity of the visual pigment molecules we will: (1) Construct translational gene fusions between the different Drosophila opsins. (2) Express the chimeric opsins in the major class of photoreceptor cells of NinaE flies using P-element mediated germline transformation. (3) Characterize the spectral and physiological behavior of the transformants in vitro and in vivo in order to identify the regions or domains of the opsin molecule involved in spectral tuning. As these studies near completion, (4) We will begin the molecular characterization of other genes encoding proteins which appear to be involved in phototransduction.